In the field of mobile communication, successors to the third-generation mobile communication system are being discussed by 3GPP, a standardization group for wideband code division multiple access (W-CDMA). For example, Long Term Evolution (LTE) is being discussed as a successor mobile communication system to W-CDMA, high speed downlink packet access (HSDPA), and high speed uplink packet access (HSUPA); and successor mobile communication systems to LTE are also being discussed. Examples of successor mobile communication systems to LTE include an IMT-advanced system, an LTE-advanced system, and a fourth-generation mobile communication system.
In LTE, orthogonal frequency division multiplexing (OFDM) is to be used as a downlink radio access method and single-carrier frequency division multiple access (SC-FDMA) is to be used as an uplink radio access method. However, in other successor mobile communication systems, a multicarrier scheme may be used as an uplink radio access method.
OFDM is a multicarrier transmission scheme where a frequency band is divided into multiple narrow frequency bands (subcarriers) and data are transmitted on the subcarriers. The subcarriers are orthogonalized and densely arranged along the frequency axis to achieve high-speed transmission and improve frequency efficiency.
SC-FDMA is a single carrier transmission scheme where a frequency band is divided into multiple frequency bands in a Fourier-transformed frequency domain and the frequency bands are allocated to different terminals. SC-FDMA makes it possible to easily and effectively reduce interference between terminals as well as to reduce variation of the transmission power. Thus, SC-FDMA is preferable to reduce power consumption of terminals and to achieve wide coverage. SC-FDMA may correspond to a variation of DFT-spread OFDM where a signal is mapped to a continuous frequency band. Use of SC-FDMA for uplink is disclosed, for example, in 3GPP TR 25.814 (V7.0.0) “Physical Layer Aspects for Evolved UTRA”, June 2006.
In a mobile communication system such as LTE, one or more resource blocks (RB) or resource units (RU) are allocated to a user device both in downlink and uplink communications. Resource blocks are shared by multiple user devices in the system. In LTE, the base station performs a process called scheduling every subframe (e.g., 1 ms) to select a user device(s) to which resource blocks are to be allocated. A subframe may also be called a transmission time interval (TTI). In downlink, the base station transmits a shared data channel using one or more resource blocks to a user device(s) selected in the scheduling. This shared data channel is called a physical downlink shared channel (PDSCH). In uplink, a user device(s) selected in the scheduling transmits a shared channel to the base station using one or more resource blocks. This shared channel is called a physical uplink shared channel (PUSCH).
In a communication system employing shared channels, it is necessary to signal (or report) allocation information of the shared channels to user devices basically every subframe. A downlink control channel used for this signaling is called a physical downlink control channel (PDCCH) or a downlink L1/L2 control channel. A downlink control signal may include, in addition to the PDCCH, a physical control format indicator channel (PCFICH) and a physical hybrid ARQ indicator channel (PHICH).
The PDCCH, for example, includes the following information (see, for example, 3GPP R1-070103, Downlink L1/L2 Control Signaling Channel Structure: Coding):
Downlink scheduling grant (information)
Uplink scheduling grant
Overload indicator
Transmission power control command bit
The downlink scheduling information may include information regarding a downlink shared channel. For example, the downlink scheduling information may include downlink resource block allocation information, identification information of user devices (UE IDs), the number of streams, information regarding precoding vectors, data sizes, modulation schemes, and information regarding hybrid automatic repeat request (HARQ).
The uplink scheduling grant may include information regarding an uplink shared channel. For example, the uplink scheduling grant includes uplink resource allocation information, identification information of user devices (UE IDs), data sizes, modulation schemes, uplink transmission power information, and information regarding a demodulation reference signal used in uplink MIMO.
The PCFICH is used to report the format of the PDCCH. More specifically, the PCFICH is used to report the number of OFDM symbols to which the PDCCH is mapped. In LTE, the number of OFDM symbols to which the PDCCH is mapped is one, two, or three. The PDCCH is mapped to OFDM symbols at the beginning of a subframe.
The PHICH includes acknowledgement/negative-acknowledgement information (ACK/NACK) indicating whether retransmission is necessary for the PUSCH transmitted via uplink. The PHICH indicates acknowledgement or negative acknowledgement for each transmission unit such as a packet and therefore can be basically represented by one bit. Since it is not efficient to wirelessly transmit each PHICH as is, PHICHs for multiple users are combined to form multi-bit information and the multi-bit information is code-division-multiplexed and transmitted wirelessly.
PDCCH, PCFICH, and PHICH may be defined as independent channels or PDCCH may be defined to include PCFICH and PHICH.
In uplink, the PUSCH is used to transmit user data (i.e., a normal data signal) and control information accompanying the user data. Also, separately from the PUSCH, a physical uplink control channel (PUCCH) is provided to transmit, for example, a downlink channel quality indicator (CQI) and acknowledgement/negative-acknowledgement information (ACK/NACK) for the PDSCH. The CQI is used, for example, for scheduling and adaptive modulation and channel coding (AMC) of the physical downlink shard channel. In uplink, a random access channel (RACH) and signals indicating allocation requests for uplink and downlink radio resources may also be transmitted as necessary.